perl：使用替换添加到路径

Question

我的脚本需要一个文件路径，并且我想将一个目录附加到路径的末尾。问题是我想不确定论证是否有尾随斜线。因此，例如：

$ perl myscript.pl /path/to/dir 
/path/to/dir/new 
$ perl myscript.pl /path/to/dir/ 
/path/to/dir/new 

我试过$path =~ s/\/?$/\/new/g，但导致双/new如果斜线存在：

$ perl myscript.pl /path/to/dir 
/path/to/dir/new/new 
$ perl myscript.pl /path/to/dir 
/path/to/dir/new 

有什么不对？

Answer 1

删除该/g修改：

$path =~ s/\/?$/\/new/ 

工作正常。
你只想修改最后添加一个“新”，所以修改器没有任何意义。

---

另外请注意，您可以使用不同的分隔符为你的正则表达式：

$path =~ s{ /? $}{/new}x; 

是一点点清晰。

Answer 2

因为/g是“全球性”，并会匹配其多次重复：

#!/usr/bin/env perl 
use strict; 
use warnings; 

#turn on debugging 
use re 'debug'; 

my $path = '/path/to/dir/'; 
$path =~ s/\/?$/\/new/g; 

print $path; 

第一替补，正则表达式引擎“左”标志“行结束”后，并不需要匹配可选/。所以再次匹配。

例如为：

Compiling REx "/?$" 
Final program: 
    1: CURLY {0,1} (5) 
    3: EXACT </> (0) 
    5: SEOL (6) 
    6: END (0) 
floating ""$ at 0..1 (checking floating) minlen 0 
Matching REx "/?$" against "/path/to/dir/" 
Intuit: trying to determine minimum start position... 
    doing 'check' fbm scan, [0..13] gave 13 
    Found floating substr ""$ at offset 13 (rx_origin now 12)... 
    (multiline anchor test skipped) 
    try at offset... 
Intuit: Successfully guessed: match at offset 12 
    12 <path/to/dir> </>  | 1:CURLY {0,1}(5) 
            EXACT </> can match 1 times out of 1... 
    13 <path/to/dir/> <>  | 5: SEOL(6) 
    13 <path/to/dir/> <>  | 6: END(0) 
Match successful! 
Matching REx "/?$" against "" 
Intuit: trying to determine minimum start position... 
    doing 'check' fbm scan, [13..13] gave 13 
    Found floating substr ""$ at offset 13 (rx_origin now 13)... 
    (multiline anchor test skipped) 
Intuit: Successfully guessed: match at offset 13 
    13 <path/to/dir/> <>  | 1:CURLY {0,1}(5) 
            EXACT </> can match 0 times out of 1... 
    13 <path/to/dir/> <>  | 5: SEOL(6) 
    13 <path/to/dir/> <>  | 6: END(0) 
Match successful! 
Matching REx "/?$" against "" 
Intuit: trying to determine minimum start position... 
    doing 'check' fbm scan, [13..13] gave 13 
    Found floating substr ""$ at offset 13 (rx_origin now 13)... 
    (multiline anchor test skipped) 
Intuit: Successfully guessed: match at offset 13 
    13 <path/to/dir/> <>  | 1:CURLY {0,1}(5) 
            EXACT </> can match 0 times out of 1... 
    13 <path/to/dir/> <>  | 5: SEOL(6) 
    13 <path/to/dir/> <>  | 6: END(0) 

这是因为$是零宽度位置锚。如果没有匹配，\/?也是如此。一旦模式一直消耗到尾部/并替换..然后正则表达式引擎继续（因为你告诉它与/g），并找到只剩$，因为这仍然是行的末尾。这仍然是一个有效的替代。

但是，为什么不改用File::Spec：

#!/usr/bin/env perl 
use strict; 
use warnings; 
use File::Spec; 
use Data::Dumper; 

my $path = '/path/to/dir/'; 

my @dirs = File::Spec->splitdir($path); 

print Dumper \@dirs; 

$path = File::Spec->catdir(@dirs, "new"); 
print $path; 

这为您提供了拆分和联接路径元素一个独立于平台的方式，并且不依赖于正则表达式匹配 - 这有各种方式就可以打破（如你找到的那个）。